This invention relates, in general, to filters and more particularly, to multiple pole filters with symmetrical transmission zeros having monolithic crystal elements.
One of the largest applications of crystal filters is in the communications industry where they are used in single sideband radios. They are well suited for this type of application because of their small size, excellent frequency stability characteristics, and low manufacturing costs. They are passive components and therefore require no power. Monolithic crystal filters are typically of the all pole type. However, occasions arise where more system attenuation is required in the stopband. In the past this has been accomplished by use of elaborate electrode patterns on the monolithic filter element itself or by using additional crystals as trap elements to provide transmission zeros. However, these techniques are costly either by increasing manufacturing costs or increasing costs from the use of additional elements.
Accordingly, an object of the present invention is to provide an improved multiple pole crystal filter.
Another object of the present invention is to provide a high order filter with symmetrical transmission zeros and low passband ripple with the use of only simple two pole symmetrical monolithic crystal filter elements and fixed capacitors.
Yet another object is to provide a filter of a desired frequency response which is obtained with simple, two pole filter elements and capacitors only, thereby giving minimum insertion loss and maximum temperature stability at a minimum cost.